Zeeman effect splitting coefficients for ClO, OH and NO in some earth atmosphere applications

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Highlights

  • We present an accurate way to calculate the Zeeman effect for ClO, OH, NO.

  • We demonstrate the effect of computing the Zeeman effect for these species for Earth applications.

  • We make recommendations for others considering Zeeman effects in Earth’s atmosphere.

Abstract

We present detailed tabulated Zeeman effect splitting coefficients for ground-state ClO, OH and NO for F ≤ 10. These species are, together with molecular oxygen, common Zeeman-affected molecules in Earth’s atmosphere. The Zeeman effect is shown to be important mostly for OH. In proposed satellite observations of mesospheric OH, a circular polarization signal of close to 3 K can be expected. ClO and NO both show more than 1% circular polarization signal.

Introduction

In a recent paper, Larsson et al. [1] compute new Zeeman splitting coefficients for molecular oxygen. This work focuses instead on three minor species of Earth’s atmosphere: chlorine monoxide, the hydroxyl radical, and nitric oxide. These molecules make up the entirerty of class 3 of the high-resolution transmission molecular absorption database (HITRAN [2], [3]): “Diatomic molecules with doublet-Π electronic state”. We are unaware of any present or past measurements of absorption lines where the Zeeman effect of these species has been studied in Earth’s atmosphere. Nevertheless, we think it is important to present a pure calculation paper because of a few recent works that could benefit from such calculations.

One of these recent proposals is by Baron et al. [4], who intend to measure the hydroxyl radical at 1.8 THz but that have yet to consider the Zeeman effect of these absorption lines. Another is by Newnham et al. [5], who want to measure a very weak hydroxyl radical absorption line at 13.44 GHz from ground-based geometry but relied on less accurate Zeeman calculations by Larsson et al. [6]. The last example is for measurements of the cosmic background radiation circular polarization in the Q-band by Petroff et al. [7] (their range is 32.3–43.7 GHz); their work use only molecular oxygen to simulate the measurements but there are OH absorption lines polarizing the Q-band as well.

This work very briefly goes over the theory relevant to compute the Zeeman effect for ClO, OH and NO in the next section. Following this section, forward simulations for the absorption lines with the strongest Zeeman effect for each of the three species in the sub-3 THz range will be presented before forward simulations of the three examples above are presented. We present 4 tables of Zeeman coefficient for F ≤ 10 in this results section. The last section concludes the study.

Section snippets

Theory and method

The molecules we consider in this work are all open-shell diatomics with one unpaired electron. Their electronic ground state is accordingly described by the term symbol 2Π, with a total spin S=1/2 and orbital angular momentum projection of |Λ|=1. The electronic ground state hosts the microwave spectral lines relevant to atmospheric spectroscopy. Fine-structure within the electronic ground state is introduced via spin-rotation interactions, spin-orbit coupling, lambda-doubling and molecular

Results and discussion

The calculated values of the Zeeman splitting coefficients for F ≤ 10 can be found in Table 2, Table 3, Table 4, Table 5 for 35ClO, 37ClO, OH, and NO respectively. The values of gJΩ±F for F ≤ 30 are shown in Fig. 1. Other gJΩ±F values can be computed as described above.

The absorption lines with the strongest circular polarization signal at 75 km limb view for each species are shown in Fig. 2, Fig. 3, Fig. 4 for ClO, OH, and NO respectively. In this scenario ClO has its strongest polarized

Conclusions

We have investigated the Zeeman effect of ClO, OH and NO in Earth’s atmospheric conditions. We have presented a method to compute the Zeeman coefficients for these species and tabulate the coefficients for low F-values in ground vibrational state. The Zeeman effect is significant for all three molecules. Attempting to measure the affected absorption lines of these species without considering the Zeeman effect at all could result in radiation errors of up to a few percentages. We have presented

CRediT authorship contribution statement

Richard Larsson: Conceptualization, Software, Writing - original draft, Visualization. Boy Lankhaar: Methodology, Software, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

None.

References (15)

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